The present invention relates to a fiber optic receiver and more particularly to such a receiver which provides predictable skewing of transitions in a binary output signal derived from the received optical signal.
As is understood by those skilled in the art, the light losses encountered in digital fiber optic transmission links can vary widely in actual practice. Losses in the cables and in the connections between the cables and optical transmitters receivers connected to the cable are subject to a variety of problems including not only the length of the signal path and mechanical changes due to temperature weathering, and the passage of time, but also assembly techniques and the skill of the technicians doing the assembly.
To assure that sufficient signal strength is available in the worst case to activate the receiver, transmitters are typically designed to produce enough signal strength to overcome the expected maximum loss. However, in the nominal case where there are relatively normal or minimal losses, the high power transmitted signal tends to overload the receiver or at least operate it at a power level which is beyond its optimum operating range.
The receiver is typically a photosensor, e.g. photodiode or phototransistor, that converts the received light into an electrical signal. Virtually all such devices are subject to saturation effects. For example, the sudden application of strong light results in a quite rapid transition to the "ON" state while the sudden removal of a strong light results in a relatively slow recovery to the "OFF" state. This effect results in a waveform skew that is strongly dependent on light intensity. The variation of skew with intensity seriously limits the data rate or signal bandwidth of systems designed to accommodate a wide range of signal path variations.
When the waveform skew is known and constant, it can be corrected by the common device of introducing a skew in the other direction and various schemes of implementing such effects are known in the art. However, since many of the causes of optical loss are unpredictable, the skewing is likewise unpredictable and unpredictable or unknown skewing cannot be compensated by the typical prior art schemes.
Among the several objects of the present invention may be noted the provision of a fiber optic receiver for digitally modulated optical signals which provides predictable skewing of transitions in a binary output signal derived from a photodetector responsive to the received optical signal; the provision of such a receiver which facilitates high data rates, hence a broad digital signal bandwidth; the provision of such a receiver which accommodates a substantial range of variation in nominal intensity of the received optical signal; the provision of such a receiver which is highly reliable and which is of simple and relatively inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.